1. Field of the Invention
This invention relates to an output circuit contained in a CMOS semiconductor integrated circuit to externally generate an output signal, and more particularly to an output circuit capable of producing an output signal whose waveform distortion is reduced.
2. Description of the Related Art
In an output stage of a semiconductor integrated circuit (which is hereinafter referred to as an IC), an output circuit for deriving out an output signal from an internal circuit of the IC is provided. The prior art output circuit which has been used in the MOS IC includes a pre-buffer circuit and an output buffer circuit. The pre-buffer circuit is formed of a CMOS inverter connected to receive a signal from the internal circuit of the IC, and the output buffer circuit includes a CMOS inverter connected to receive an output signal from the pre-buffer circuit. The current driving ability of the output buffer circuit is set sufficiently larger than that of the pre-buffer circuit. For example, the channel widths of MOS transistors constituting the output buffer circuit are set larger than those of MOS transistors constituting the pre-buffer circuit. An output signal of the output buffer circuit is supplied externally as an IC output signal from the IC.
It is well known in the art that, in operation of the output buffer circuit or at the time of switching operations of the MOS transistors constituting the output buffer circuit, distortion due to the overshoot or undershoot occurs in the waveform of the output signal. The waveform distortion in the output signal is caused by resistive, capacitive and inductive loads associated with power source lines for supplying a power source voltage and output lead wires for transmitting the output signal. With the enhancement of the operation speed of the IC and increase in the output current of the IC, it is required to attain, in the CMOS output circuit, the delay time characteristics and large output current which can be attained when a signal is transmitted in the Schottky TTL. In order to meet the requirement, it is necessary to increase the mutual conductances of the MOS transistors constituting the output buffer circuit. When the mutual conductance of the MOS transistor is increased, the waveform distortion in the output signal becomes large and cannot be neglected.
In the prior art, in order to suppress the waveform distortion in the output signal to a minimum, a superhigh-speed diode or an inductor formed of ferrite beads called EMI-FIL is externally connected to the signal output terminal of the IC. However, use of the diode and EMI-FIL results in high cost, increase in the number of manufacturing steps to thereby raise the manufacturing cost, and increase in the occupation area. As a result, the cost of the printed circuit board on which the IC is mounted becomes high.